Pressure onion
In soil mechanics , the pressure bulb is a specific form of pressure distribution that occurs under an external load. The pressure onion is important in construction, agriculture and forestry.
“In the subsoil, which is not fully elastic isotropic (the soil only follows Hooke's law approximately ), isobars acquire a more or less elongated shape; the so-called pressure onion forms. "
Calculations of the pressure bulb are never exact, as the concentration factor of the subsoil is only estimated and cannot be precisely determined. The deviations are usually around 30%, as a concentration factor of 3–4 can be assumed. Usually the value 3 is assumed.
definition
In the case of stiff , but not particularly tensile materials such as brickwork and unreinforced concrete, it is assumed that a point load only affects a triangular or conical area below the point of application, the boundary of which does not deviate by more than about 30 ° from the vertical. In the case of elastic or plastic materials, on the other hand , shear stresses can be compensated for by deformation, so that a circular or elliptical pressure distribution is established.
The pressure bulb describes the representation of the isobars in elastic-plastic solids. The sectional view shows a cross section through a solid with at least one free surface. A force acts on the surface, the surface of which is small compared to the dimensions of the solid. In the sectional image, lines are drawn through points at which the same pressure is established due to the action of the force, the isobars. If the isobars are circular or elliptical, this representation is called pressure onion.
The pressure is a scalar , i.e. directionless, quantity. The illustration therefore does not necessarily indicate in which direction the forces act. The isobars to be drawn are expediently selected at equidistant pressures so that, similar to the contour lines on a topographical map, an image of the increase and decrease in pressures due to the density of the isobars is expressed.
Importance in agriculture and forestry
Agricultural and forestry machines drive over the ground with pneumatic tires. In order to increase efficiency, the machines are sometimes larger, but also heavier due to additional functions. This leads to increased area loads and consequently to soil compaction . Above certain limits, soil compaction, for example in arable farming, is not reversible. In the forest, in addition to the numerical soil compaction, the soil ecosystem can also be damaged. Agriculture and forestry therefore try to record, calculate and minimize the forces that arise in the subsoil through traffic.
When measuring and simulating the spread of pressure under a vehicle, the representation in the form of a pressure bulb is very helpful. With well-chosen scales it can be seen at a glance how damaging z. B. acts a certain tire width together with a load on the ground. The pressure bulb shows how quickly the pressure is distributed and what maximum values it reaches.
In civil engineering
The calculation or measurement of pressures under structures and their graphic representation as a pressure bulb provides information about the condition of the soil, its current compaction and the compaction that is still possible due to additional pressure. This is of practical importance, for example, when new structures are to be erected in the vicinity of existing structures. The depiction of the pressure bulb under existing structures shows the uneven compression of the ground in their surroundings. Such uneven compaction can lead to uneven and therefore harmful settlement of new structures in the neighborhood. The graphic representation of the pressure bulb is therefore a particularly clear way of recognizing such problems and taking countermeasures.
Use in other sciences
In road construction, pressure considerations are usually different than in arable farming, since most road building materials follow Hooke's law and therefore no pressure bulbs build up. Similar representations are used in mechanical engineering and related sciences, but the term pressure onion is not used here.
Web links
Footnotes
- ↑ Konrad Simmer : Foundation 1 - Soil Mechanics and Earth Static Calculations. 19th edition. BG Teubner, Stuttgart 1994, ISBN 3-519-45231-6 , p. 170.
- ↑ Otto Karl Fröhlich : Pressure distribution in the subsoil. Springer Verlag, Vienna 1934.